1. Field of the Invention
This invention relates generally to dilatometers and, more particularly, to dilatometers having multiple linear variable differential transformer dilation sensors for making independent measurements on multiple specimens which are subjected to identical temperatures and rates of temperature change within a common furnace. Such multiple sensor dilatometers can handle large numbers of specimens in a short period and are especially suited for production control purposes.
2. Description of the Prior Art
Dilatometers are analytical instruments that respond to the linear thermal expansion or contraction of solids. Typically, these instruments have a variable temperature furnace in which the test specimen rests between a flat surface on a stationary object and an opposing flat surface on a movable object, such as a ceramic pushrod, that extends outside the furnace. Temperature induced changes in the length of the specimen are transmitted through the rod to a mechanical, optical or electrical system for amplifying and measuring that change. These instruments can be used to make precise measurements of changes in length resulting from small temperature changes or to plot variations in the rate of linear expansion or contraction over a broad temperature range.
Among the least sophisticated dilatometers in common use are those in which the push rod is coupled to a dial gauge and the dilation of a specimen is read directly from that gauge. Such dial gauge dilatometers are simple to use and inexpensive, but generally are suitable only for low to moderate temperature applications that do not demand great precision.
U.S. Pat. No. 3,680,357 describes a far more precise type of dilatometer in which the dilation sensor is a linear variable differential transformer which translates specimen dilation into electrical signals that can readily be amplified and recorded. In such sensor, the core floats freely in the coil and each of these elements is separately supported at its ends by a pair of compound cantilevered flat springs. These springs permit independent and frictionless axial movement of the suspended element, but restrain radial or transverse movement. This independent and frictionless axial mobility of the core and coil facilitates calibration of the sensor and renders it extremely sensitive to minute changes in specimen length, thereby making possible exceptionally accurate measurements of thermally induced expansion or contraction.
When such dilatometer is used with a single pushrod, as shown in FIG. 1 of the aforementioned Patent, that pushrod commonly is coupled to and supported only by the core of the linear variabe differential transformer and it extends into the open end of a tubular specimen holder, where it abuts a specimen that is held between a flat ground surface at the end of the pushrod and a similar flat ground surface on the interior of the other closed end of the specimen holder. An opening commonly is provided in the wall of the specimen holder adjacent to its closed end to facilitate specimen changes. The closed end of the specimen holder is inserted into a variable temperature furnace, which, for many applications, is a conventional electric tube furnace.
For measurements of the differential thermal expansion of two specimens, separate closely spaced pushrods may be coupled to the core and coil of the linear variable differential transformer and the equally closely spaced specimens are held abreast within a single tubular specimen holder that is inserted in a similar electric tube furnace, as shown by U.S. Pat. No. 3,898,836.
Since a significant amount of heat is lost through the open ends of the oriface of an electric tube furnace, it is usually advantageous to have an oriface of as small diameter as practical in order to minimize furnace temperature gradients and power consumption. The use of such small oriface furnace does, however, restrict the maximum diameter of the specimen holder, which, in turn, limits the maximum diameter of both the specimen and pushrod. The maximum diameter of the specimen holder may be further restricted when furnace temperatures are very high and it becomes necessary to protect the specimen and sensor from oxidation by placing them within an evacuated enclosure. Typically, such evacuated enclosure includes a closed end protective tube that surrounds the specimen holder and that also must fit into the oriface of the electric tube furnace.
In order to take advantage of the moderate acquisition cost, internal temperature uniformity and economy of operating an electric tube furnace having a small oriface, dilatometer specimens, pushrods, specimen holders and protective tubes generally have been reduced to as small diameter as is practical without sacrificing structural integrity and ease of handling. In the case of two pushrod differential dilatometers, another practical limitation on diameter reductions of the specimen holder and protective tube has been the distance between the pushrod couplings on the core and coil of the linear variable differential transformer.
Largely as a result of such diameter reductions, highly accurate and power efficient dilatometers now are available which employ linear variable differential transformer sensors and conventional electric tube furnaces having small orifaces. These are widely used for research purposes and are invaluable in studying the compatibility under changing temperature conditions of different materials which are bonded together or are in close contact; e.g., metal to glass, enamel to substrates, thin film deposits in microcircuits or metal or plastic fillings in natural teeth. They also have been utilized to detect phase changes occurring during the heating or cooling of materials, such as steel, and to study the effect of different heating or cooling rates on the physical properties of those materials.
They are, however, not as widely utilized for production control or other applications that involve repetitive measurements with a single pushrod sensor on large numbers of specimens which must be subjected to identical temperatures and rates of temperature change. Since an individual measurement may require up to several hours or more, a conventional single sensor dilatometer and furnace, which measures the dilation of a single specimen at a time, simply is too slow for most production control requirements. The use of multiple single sensor dilatometers with individual furnaces is unattractive both because of cost and the difficulty of identically controlling different furnaces. The use of multiple single sensor dilatometers with a single furnace is similarly unattractive because an electric tube furnace having an oriface large enough to accomodate the multiple protective tubes and specimen holders of such dilatometers is otherwise unnecessarily large and its excessive internal temperature gradients make it very difficult to subject the widely spaced specimens to identical temperatures and heating rates.
It is the broad object of this invention to provide an dilatometer that is capable of concurrently measuring the thermal expansion or contraction of multiple specimens within a single energy efficient furnace, thereby making it suitable for production control purposes. A specific object of this invention is to provide such multi-sensor dilatometer that employs linear variable differential transformer dilation sensors, each of which has a single pushrod coupled to its coil. It is a further specific object to provide such dilatometer in which the coils of at least two sensors are closely positioned abreast and are coupled to separate closely positioned parallel pushrods which are adapted to abut separate specimens held within a single specimen holder. Another specific object is to provide such dilatometer in which at least four specimens can be uniformly heated within an electric tube furnace having a small diameter oriface and internal temperature gradient. Still another specific object is to provide a compact unitary dilatometer measuring head having multiple linear variable differential transformer dilation sensors that are closely positioned abreast, that can readily be calibrated and in which the coils exhibit independent frictionless axial mobility, but are laterally restrained.
These objects and other advantages which will be apparent from this specification, are achieved by the invention described below.
It has now been determined that accurate measurements of dilation can be made with a single pushrod that is coupled to the coil, rather than the core, of a linear variable differential transformer sensor and that such coupling permits exceptionally close spacing of the pushrods from multiple sensors. Broadly, my invention is an energy efficient single furnace dilatometer having multiple linear variable differential transformer dilation sensors in which the pushrods are coupled to closely spaced coils and which, therefore, is capable of concurrently measuring the thermal expansion or contraction of multiple closely spaced specimens under substantially identical conditions in a single energy efficient furnace.
One aspect of this invention is such dilatometer comprising
(a) a measuring head in which at least two linear variable differential transformer dilation sensors are positioned in a closely spaced apart cluster so that each sensor is parallel to, abreast of and adjacent to each other sensor, the coil of each sensor is axially movable by and supports an end of a separate tandemly positioned pushrod, the other end of that pushrod is adapted to abut a flat surface on a separate specimen, and each pushrod and its supporting coil is coupled at a position on that coil closest to the axis of the cluster of sensors so that all the pushrods are positioned parallel, abreast and closely spaced apart,
(b) a tubular specimen holder having an open end, into which the specimen abutting ends of the pushrods extend, and a closed end that is internally adapted to abut another flat parallel surface on each specimen and
(c) an electric tube furnace into which the closed end of the specimen holder extends.
Preferred embodiments of this aspect of the invention include dilatometers in which from two to four sensors are employed in the measuring head and dilatometers in which two measuring heads are employed with their associated push rods and specimen holders extending into opposite ends of the oriface in an electric tube furnace.
Another aspect of this invention is a measuring head for such dilatometer which has at least two linear variable differential transformer dilation sensors with axially movable coils. These sensors are positioned in a closely spaced apart cluster in which each sensor is adjacent, parallel and abreast of each other sensor and each coil bears a separate coupling means which is positioned on that coil proximate to the axis of the cluster of sensors and which is adapted for engaging an end of a separate pushrod and supporting that pushrod parallel to and in tandem with its coupled coil.
Preferred embodiments of this aspect of the invention include measuring heads having two to four sensors, measuring heads in which the cores of the sensors are attached to a single yoke on which a single micrometer bears to axially move all of the cores for calibration purposes, measuring heads in which each core is supported by a pair of compound cantilevered flat springs, measuring heads in which each core is moved axially by a separate calibrating micrometer and measuring heads in which each core is supported by a separate calibrating micrometer.
An especially preferred embodiment of this aspect of the invention is a dilatometer measuring head in which the core floats freely in the coil of each of two linear variable differential transformer dilation sensors that are positioned parallel, abreast and closely spaced from each other, each core is axially movable by a calibrating micrometer, each coil is supported by and axially movable on a pair of compound cantilevered flat springs and each coil is attached to a separate coupling means that is positioned on that coil proximate to the other coil and that is adapted to engage an end of a separate pushrod and support that pushrod parallel to and in tandem with the coil to which it is coupled.